The present invention relates to a spring-loaded piston for spring-loaded brake cylinders, particularly for brake cylinders of commercial vehicles.
Service brake cylinders, spring-loaded brake cylinders as well as combined service brake and spring-loaded brake cylinders, frequently called xe2x80x9ccombination cylindersxe2x80x9d are known from practice. Brake cylinders of this type are used for the operation of brake systems of vehicles, particularly of commercial vehicles. In the case of known pure spring-loaded brake cylinders, a prestressed preloaded spring acts by way of a spring-loaded piston upon a piston rod, so that a brake can be operated in an application. The service brake part of a service brake cylinder or of a combination cylinder, in the case of brake cylinders known from practice, generally contains a diaphragm which can be acted upon by compressed air and which acts with respect to a plate and an operating tappet connected with the plate. In the case of a combination cylinder, the spring-loaded brake part is saddle-mounted on the service brake part. Such a saddle-mounted spring-loaded brake part is described, for example, in German Patent document DE 198 30 154 A1. Here, a piston rod of the spring-loaded brake part, which is connected with the spring-loaded piston, acts upon the diaphragm of the service brake cylinder when actuated by the preloaded spring and transmits via the plate of the service brake cylinder, the spring-loaded brake force upon the operating tappet. For this purpose, the preloaded spring is tensioned beforehand when the spring-loaded chamber is acted upon by compressed air.
Such spring-loaded brake cylinders have a spring-loaded piston cast of aluminum or zinc. However, these cast components have the significant disadvantage that, after the casting, finishing steps are always required for the removal of possible burrs or for evening-out the surfaces. Furthermore, frequently required bores for fastening devices have to be made, for example, in a cutting manner, or seat surfaces for sealing rings or slide rings or the like have to be worked-in. Also, these spring-loaded pistons made of aluminum or zinc have the disadvantage that, as a result of not completely avoidable moisture, they are subjected to corrosion or similar aging processes, which may finally lead to a malfunctioning. Finally, the manufacturing of spring-loaded pistons as an aluminum or zinc casting is relatively expensive, and it is another disadvantage that the casting molds are subjected to high wear, and therefore only limited quantities can be produced.
Spring-loaded pistons made of aluminum or zinc castings in accordance with known practices have the additional disadvantage that only limited forces can be introduced by the preloaded spring into the spring-loaded piston and be transmitted by the spring-loaded piston to a piston or a piston rod because, as known, aluminum or zinc castings can be placed in tension stress only to a limited extent.
It is therefore an object of the present invention to provide a spring-loaded piston which can be produced at lower cost without any significant finishing expenditures. In addition, it should be possible to minimize the above-mentioned corrosion and aging risks.
It is another object of the present invention to provide suitable devices so that either higher forces than previously could be transmitted from the preloaded spring into the spring-loaded pistons and from the latter to the piston rod, or, while the forces remain the same, the wall thickness of the force-loaded wall sections of the spring-loaded piston can be reduced.
These objects are achieved by a spring-loaded piston for a spring-loaded brake cylinder, particularly of commercial vehicles, wherein the spring-loaded piston is made of plastic.
According to the invention, a plastic is selected as the material for producing the spring-loaded piston. Contrary to existing prejudices with respect to plastic as a material considered useless for spring-loaded pistons in comparison to aluminum and zinc, tests have surprisingly shown that spring-loaded pistons made of plastic can have operating characteristics which are just as good as those of conventional spring-loaded pistons. Also surprisingly, the braking forces achieved thereby are no less than the braking forces achievable using the conventional spring-loaded pistons. Furthermore, the expected useful life of a spring-loaded piston made of plastic is against all expectations even above the expected useful life of a conventional spring-loaded piston.
In that a spring-loaded piston made of plastic is suggested for the first time, which can be made, for example, as a plastic injection molded part, the clearly higher quantities that can be made by a plastic injection mold can be advantageously utilized for increasing productivity, in which case rates of increase of 10 to 20 times in comparison to the conventional aluminum or zinc castings can be implemented without additional expenditures. Furthermore, when the spring-loaded piston is made of plastic, a high-expenditure finishing can, as a rule, be eliminated. Plastic injection molded parts have smooth surfaces from the start. Burrs, which have to be removed subsequently at high expenditures, normally do not occur in the case of plastic injection molded parts. The manufacturing costs of a spring-loaded piston made of plastic can therefore clearly be reduced in comparison to the manufacturing costs of conventional spring-loaded pistons cast of aluminum or zinc.
Furthermore, the first-time selection of plastic as the material for producing the spring-loaded piston according to the invention has the additional advantage that a spring-loaded piston made of plastic has greater useful life and is resistant to corrosion that may occur because of possibly existing moisture.
In addition, the use of plastic for producing the spring-loaded piston also has the additional advantage of a clear weight reduction in comparison to the aluminum or zinc casting technology. This reduces the expenditures with respect to handling during the manufacture of a spring-loaded piston produced as an injection molded part of a plastic and contributes to lower transport costs of the produced parts because of their smaller measurements.
Furthermore, a spring-loaded piston made of plastic has the additional advantage that seats for sliding rings, seals, or the like have a sufficiently high surface quality and/or fitting precision so that the working-in of such fitting surfaces for sliding rings, seals or the like, which is expensive specifically in the case of conventional aluminum or zinc castings, can be eliminated. This, in turn, is reflected in reduced machining time as well as in lowering of costs.
Not least, the manufacturing of a plastic spring-loaded piston according to the invention permits a noticeable noise reduction during the further machining since its hollow plastic body does not produce any significant noise in contrast to a metallic hollow body. This circumstance is particularly noticeable by the workers entrusted with the integration of the brake cylinder, in that the workplace quality is improved.
Advantageous further developments of the invention are described herein.
Thus, in a preferred embodiment, a spring-loaded piston according to the invention has an insert, which is preferably molded in the same process into the wall or into wall sections of the spring-loaded piston and which is made, for example, of steel or a similar suitable material which can be well stressed with respect to tension without having significant deformations. This reinforcing insert or steel insert is largely integrated in the wall of the spring-loaded piston made of plastic and is therefore surrounded by plastic. The steel insert is advantageously used for the reinforced transmission of the force introduced by the preloaded spring into the spring-loaded piston to its transfer area for the introduction or transfer into the piston rod, in which case the steel insert can be shaped in an optimal manner according to its mechanical loads. In addition, such a reinforcing insert or steel insert has the advantage that the wall thickness of wall sections, which are under tension, of the spring-loaded piston according to the invention can clearly be reduced in comparison to conventional spring-loaded pistons made of aluminum or zinc castings.
According to a preferred embodiment, the insert extends from a piston plate section, particularly from a force introduction area into the spring-loaded piston, which is adjacent to a contact surface of a preloaded spring, to a force transfer area from the spring-loaded piston to a piston or a piston rod, which force transfer area is arranged in a ring-shaped piston center. In this case, the insert may extend in this section, for example, in a full-surface ring-shaped surrounding manner, but it may also be embedded in the respective wall section as mutually spaced, radially extending strips. Likewise, it is possible to construct the insert similar to a crown with a ring and adjoining rays and to mold it into the wall.
In another preferred embodiment, the reinforcing insert is curved in an arched manner in the force introduction area of the preloaded spring, then extends in a relatively straight line (or inclined at a slight angle) ring-shaped coaxially to the longitudinal axis X, and then bends away in one or several sections to a ring-shaped end section oriented essentially perpendicular to the longitudinal direction, against which end section the piston rod rests for receiving the force. In this case, the piston rod is preferably centered in its position by a corresponding nose which engages the inside of the piston rod. In a preferred embodiment, the insert has a shoulder for this purpose, the spring-loaded-piston-side radially interior end of the piston or of the piston rod resting against this shoulder.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.